The Universal Postal Union (UPU), an agency of the United Nations, is an intergovernmental organization that establishes rules for the exchange of documents and parcels among national post offices. The primary international agreement governing international postal shipments is the Universal Postal Convention, which is revised and readopted every four years in a genral "Congress" of all member countries. The next Congress of the UPU willl be held in Istanbul in September 2016 . See genrally http://www.upu.int/en.html.